The present invention relates to a fuel injection valve for use in a direct-injection type internal combustion engine.
In a conventional diesel engine, there is a problem that while the fuel injection rate, i.e., injection quantity per unit time should be set at a suitably high value for improvement of the output characteristic of the engine as well as prevention of the emission of nitrogen oxides and other detrimental products, an increased fuel injection rate can lead to a correspondingly shortened injection period, which can cause a reduction in the combustion duration, resulting in a combustion noise, and can also cause an increase in maximum pressure within the engine cylinders.
On the other hand, in a conventional fuel injection valve used in such conventional diesel engine, the nozzle needle is urged by a single spring, so that the valve operation is monotonously affected by pressure changes within the injection pipe connected to the valve, that is, a specific amount of increase in the pressure within the injection pipe causes the same amount of increase in the lift of the nozzle needle in a lower injection quantity range as in a higher injection quantity range. Therefore, in a lower injection quantity range, the nozzle needle can lift through an excessive stroke to provide an excessive injection quantity due to an increase in the pressure within the injection pipe, and the resulting pressure drop in the injection pipe causes a too small injection quantity during the next injection stroke, followed by an excessive injection quantity during the further next injection stroke. In this manner, the fuel injection valve suffers an unstable or irregular injection. Although a conventional fuel injection valve using a pintle nozzle can overcome such phenomenon of unstable or irregular injection due to the shape of its nozzle needle which produces a throttling effect, one using a hole nozzle is unable to avoid the abovementioned phenomenon.
To solve this problem, a fuel injection valve has been proposed by the assignee of the present application in U.S. Ser. No. 237,941 filed Feb. 25, 1981 now U.S. Pat. No. 4,359,191, issued Nov. 16, 1982 for instance, which includes a second nozzle spring arranged in its nozzle holder in addition to a conventionally employed first nozzle spring. In this proposed fuel injection valve, fuel injection takes place in two steps, i.e., an initial injection and a main injection, in such a manner that during the initial injection the first nozzle spring operates wherein the nozzle needle lifts through a limited stroke, and the main injection subsequently takes place with the valve opening pressure determined by the combined force of the first and second nozzle springs. This double-step injection reduces the injection rate throughout the whole injection period, thus substantially overcoming all the aforementioned drawbacks of combustion noise, emission of nitrogen oxides and irregular injection.
According to the above proposed fuel injection valve, during the initial injection lifting of the nozzle needle causes corresponding lifting of a first movable spring seat to cause contraction of the first nozzle spring to execute an initial injection lift, and during the subsequent main injection the first movable spring seat is further lifted to push a rod-like second movable spring seat to cause contraction of the second nozzle spring together with the first nozzle spring to execute a main injection lift.
The valve opening pressure for initial injection is determined by the setting load of the first nozzle spring for initial injection, and the valve opening pressure for main injection by the sum of the setting loads of the first nozzle spring and the second nozzle spring for main injection, respectively.
However, according to this proposed fuel injection valve, adjustment of the setting load of the first nozzle spring is carried out by selecting the thickness of a shim interposed between the same spring and its seating surface formed in the nozzle holder, which requires dismantling the injection nozzle portion of the valve each time the initial injection valve opening pressure and the main injection valve opening pressure are adjusted. Further, according to the proposed valve, to adjust the initial injection lift, an adjusting threaded member is rotated to displace the second movable spring seat engaged therewith to vary the gap between the first movable spring seat and the second movable spring seat. However, once the initial injection lift gap has been adjusted, it is difficult to ascertain whether or not the adjusted gap has an accurate value. In addition, like ordinary threaded parts, small tolerances exist between the thread formed on the outer peripheral surface of the adjusting threaded member and the thread formed on the associated inner peripheral surface of the nozzle holder, which may result in play of the threaded member relative to the nozzle holder. This play of the threaded member makes it difficult to make accurate adjustment of the initial injection lift gap.